Ocular melatonin contents in a cyprinid teleost, oikawa Zacco platypus were determined by newly-developed radioimmunoassay. Under light-dark (LD) cycles of 12:12 hr, melatonin levels in the oikawa eye exhibited daily rhythms with higher levels during the dark phase than those during the light phase. However, the rhythms disappeared under either constant light (LL) or constant darkness (DD). Ocular melatonin contents under LL remained at low levels while those under DD kept high titers. These results indicate ocular melatonin rhythms in oikawa are driven not by a circadian clock but by LD cycles.